1. Field of the Invention
The present invention is directed to a top loaded antenna unit, and more particularly to a top loaded antenna unit which is mounted in a concealed manner within a vehicle for mobile transmission.
2. Description of the Prior Art
In recent years, top loaded antennas have become of interest for use in mobile transmissions due to their superior nondirectional characteristic and low profile structure. The top loaded antenna is mostly required to be not visible, when mounted in a vehicle, so as not to mar the appearance and not to be noticed or fooled by a burglar. For this purpose, it is a general practice to mount the antenna within a portion of the vehicle adjacent a front or rear windshield below a dashboard or the like covering made of dielectric material. When so mounted, there arises a problem that resonance frequency of the antenna will shift by .DELTA.f.sub.1 to a lower side from an intended frequency f.sub.0 due to the influence of the adjacent dielectric material, as shown in FIG. 4, and the shifting amount will be greater as the antenna is closer to the covering of the dielectric material. In order to compensate for the low-going shifting of resonance frequency, it is required to set a resonance frequency at a level greater than an intended frequency at the time of shipping the antenna. However, this scheme is not satisfactory and fails to achieve precise compensation or adjustment of resonance frequency to an intended frequency since the shifting amount will vary indefinitely. This is because the dielectric material used will differ from different vehicles and also because the antenna will be spaced at varying distances from the covering due to different structural limitations in different types or models of the vehicles.